In today's widely deployed wireless communications networks, such as the Long Term Evolution (LTE) wireless data networks developed by members of the 3rd-Generation Partnership Project (3GPP), the wireless terminals, referred to in 3GPP documentation as “user equipments,” or “UEs,” are not configured to perform beamforming in uplink (wireless-terminal-to-base-station) transmissions. This is anticipated to change in so-called fifth-generation (5G) wireless networks, where at least some wireless terminals will be capable of beamforming in uplink transmissions, using multiple transmitter chains driving multiple antenna elements.
It will be understood that beamforming, as a general matter, is the combination of radio signals from a set of small non-directional (or low-gain) antennas to simulate a large directional antenna. By controlling the phase and amplitude of the signal at each antenna, the resulting antenna pattern may be shaped and steered electronically, to create a certain beam width or to maximize its gain in a desired direction.
As a general matter, beamforming can be analog or digital, with the former utilizing analog phase shifters to create phase differences between the signals supplied to each antenna element and the latter using digital techniques to create the needed phase shifts of the transmitted signal at the several antenna elements of the transmitter array. Digital beamforming techniques are expected to be used in 5G wireless devices. Several transmitter (TX) chains are needed, with each transmitter chain in a digital beamformer including a digitally controllable signal source feeding a high-power amplifier, which is then coupled to one of several transmitter elements in an array. The antenna elements together form an antenna aperture. The more transmitter chains used to drive the antenna aperture, the narrower the beam can be.
In each transmitter chain, the phase shift and amplitude of the signal driving the antenna element can be individually controlled. The beamforming is achieved by using several transmit chains and controlling the phase and amplitudes of the signals transmitted from each chain so that they add up constructively in one direction, while adding up destructively in other directions. This results in an overall antenna gain in some directions and a loss in other directions, especially when there are more than two transmitters. FIG. 1 illustrates an example transmit beamformer with four antenna elements—as seen in the figure, each antenna element is fed by a high-power amplifier, with each high-power amplifier in turn being fed by a complex-weighting element, which applies a phase shift θk and an amplitude weight ak to a copy of the signal to be transmitted. Together, the phase shift and amplitude weight constitute a complex-valued weight wk. In a digital beamformer, this phase shifting and amplitude weighting is performed digitally, e.g., before upconverting a baseband version of the weighted signal to a radio frequency signal for amplification and transmission.
The benefits of beamforming are manifold, and include enhanced coverage, longer achieved ranges using the same output power, and, in many cases, less multipath at the receiver side. The probability of interference towards other radio links can be significantly reduced, as the wireless terminal does not transmit significant energy in directions other than the direction of the intended receiver. This also increases efficiency, as the transmitted signal is not wasted in the form of interference towards other receivers in other directions.
While uplink beamforming can be used to provide significant performance gains, a key tradeoff is the power consumption of the device. Each distinct transmit chain in the beamforming device comprises a power amplifier and possibly comprises additional analog components, such as a mixer and local oscillator amplifiers. Each of these components consumes extra power, some of which is independent of the signal amplitude produced by that chain. As a result, even if the total output power from the combined transmitter is the same as when only one transmitter is used, the total power consumption when using several transmitter chains can be substantially higher. Thus, a narrow beam, produced by combining signals from several transmitter chains, requires a higher power consumption than a non-directional beam with the same transmitted power.
Another problem with narrow beams is that they are more sensitive to changes in the orientation of the wireless device, as compared to wider or non-directional beams.